Yes, you are correct that if you have a track running at one level, with no effects or gain adjustments or panning routes to a bus, the bus shows the same level.
You are missing something.
There is an input gain on the track and the bus.
Any effect, in the FX Bin or the Pro Channel could change the gain.
You can rule out FXs by clicking E to bypass all FXs.
Once that's done, there should only be the track routed to the bus (it's volume fader, gain and pan) and input gain on the bus (fader and pan can impact output level but not input level - you can set the meter for either).